1. Field of the Invention
The invention relates to a measuring device for determining the concentration of low-molecular alcohols in water or acids.
2. Description of Related Art
Methanol, a low-molecular alcohol, represents an attractive fuel for electrochemically producing energy in fuel cells. An alkaline liquid electrolyte in particular a potassium hydroxide solution, can be used in fuel cells operated with methanol. However, due to the formation of carbon dioxide during the oxidation of methanol, fuel cells with an acidic electrolyte, such as sulfuric acid, are preferred. Such an electrolyte is very aggressive, especially at elevated temperatures, so that only relatively expensive materials can be used to construct batteries of this type.
For some time, so-called PEM fuel cells have been developed, i.e., fuel cells having a polymer electrolyte membrane (PEM). This cell type is also suited for constructing fuel cells used for the direct conversion of methanol, so-called direct-methanol fuel cells. In this case, the methanol fuel no longer has to be present in an aggressive electrolyte, such as sulfuric acid, rather aqueous methanol solutions can also be used.
In PEM fuel cells operated with methanol--due to the loss of methanol by diffusion through the polymer electrolyte membrane--the methanol concentration at the anode must be adapted as best possible to the existing electrical power level of the fuel battery. For this purpose, a concentration-measuring cell is needed, i.e., a measuring device for determining the concentration of the methanol, which serves as a controlling and monitoring unit in the fuel circuit.
Customary measuring methods for determining the concentration of alcohols are relatively inaccurate or very expensive in the concentration range of 0 to 2 mole/l--due to the only slightly changing physical properties. Electrochemical measuring cells for determining the concentration of alcohols presuppose acidic or alkaline electrolytes, which is likewise disadvantageous (see G. Ciprios, "Proceedings of the 20th Annual Power Sources Conference", May 24-26, 1966; "Energy Conversion", vol. 12 (1972), pp. 65-68).